Binaural Recording Explained (article sample)
'Introduction to binaural sound recording' ‘Binaural’ is an unfamiliar term to many people, and often is missused when describing stereo recording. In brief, binaural recording involves placing omnidirectional mics on an object designed to replicate the human head (or in some cases even microphones can be placed in the ears of a real humans head). Common binaural recording techniques include jecklin disc, dummy head as well as “binoral microphones” The results can be very impressive creating life like stereo images in which the listener can percieve sounds location and distance, making it seem like a sound source has come from clearly-defined location within the listeners environment. The drawback of binaural reproduction is that the effect of realism only works when the listener is using headphones. 'What is the difference between a stereo recording and a binaural recording?' To demistify the subject here are some quick definitions of different audio formats: *Mono: one channel of audio signal *Stereo: two different channels of audio signal, recorded with two microphones spaced apart (or with a single stereo microphone) *Dual mono: two channels of the same audio signal coming from the same microphone *Binaural=two different channels of audio, recorded on either side of a human or artificial head, preferable in the ears. "Stereo" recordings are essentially any recording made with two channels of audio, where the signal on each channel is different. In contrast, "dual mono" is also two channels of audio, but where the signal on each channel is the same. Either of these recordings can be made with Omnidirectional or Cardioid microphones. In a stereo recording, when you record something with two microphones spaced some reasonable distance apart, you get slightly different sound waves hitting each mic, resulting in different sounds recorded in each channel. When you play the recording back, you hear a sense of space between the speakers (or headphones) which creates the stereo image. "Binaural" recordings are two channel recordings created by placing two omnidirectional microphones inside, or as close to the ears as is practical or on a apparatus to replicate the human head. Using this technique, the head and ear structure affect the way sound waves are picked up by the microphones so that the location information contained in the frequency, amplitude and phase responses of the left and right channels closely match the cues required by the human auditory system to localise sound sources. Positioned in this way, the microphones accurately capture sonic information coming from all directions and will produce extremely realistic recordings when listened to through headphones. There are three inter-channel differences which may be involved in stereo recording and reproduction. In everyday life our sense of hearing uses all of them as cues to locate where sounds are coming from, only they’re referred to as “interaural differences”, meaning the differences in sound percieved by each ear. The first consists of simple differences in sound intensity in each stereo channel, or in the sound reaching each ear. The second involves the miniscule time delay ''between sound waves from an off-centre source reaching one ear and then the other. The third involves changes in the spectrum of frequencies'' reaching each ear. These changes are caused by the mass and size of the head, by parts of the upper body, and by the shape of the outer parts of the ears. We’re aware of off-centre sounds being louder in one ear than another, however interaural differences in time of arrival are processed by our brains in a fast and automatic way, below the level of conscious experience. Assuming our listener is at sea level and the air is at room temperature, a sound from one side will reach the facing ear around 0.6 milliseconds before reaching the other ear, due to the 20-odd centimetre gap between the two. That the brain can detect such a brief delay is particularly impressive considering how the fastest nerve signals are propagated at only a third of the speed of sound. When there’s a difference between the spectrum of frequencies reaching each ear, that too is processed under the radar screen of consciousness. Low-frequency sounds have longer wavelengths which can pass around her head unimpeded, but high-frequency sounds have much shorter wavelengths so they’re partially blocked and filtered. Frequency differences start to become an especially important cue for sound localisation where wavelengths are shorter than the 20 centimetre-odd diameter of the human head, which is from around 1.6 kilohertz upwards. There are various ways of setting up mics to record in stereo. Some techniques use inter-channel differences in loudness or sound intensity alone. Others place two directional mics at an angle to one another and with a gap between them ranging from around 17 cm to 30cm. This captures differences in sound intensity and time of arrival between the mics from off-centre sound sources. Recordings with the most realistic-sounding stereo images use all three cues of intensity, time of arrival, and the spectrum of frequencies. In this way they try to get close to how our sense of hearing detects where sounds are coming from. Such techniques include placing an obstacle or baffle between two mics, and binaural recording where two mics are placed in the ears of the recordist or in the ears of dummy head. Binaural Recording Techniques: There various approaches to binaural recording, however commonly used methods are: Dummy head recording Dummy head recording (also known as artificial head or Kunstkopf) is a method used to make binaural recordings, that allow a listener wearing headphones to perceive the directionality and the room acoustics of single or multiple sources. True dummy head recording uses an artificial model of a human head, built from selected acoustic materials to emulate the sound-transmitting characteristics of a real human head, with two microphone inserts embedded at the “eardrum” locations, Simulated dummy head recording processes a signal electronically to imprint the HRTF (head related transfer function) information associated with a specified direction. Jecklin/ schnieder disk This microphone system with a separation baffle between omnidirectional microphones, has been known for years as the Jecklin disc (disk), and was developed by Jürg Jecklin, a Swiss sound engineer. The sound-absorbing "separation-disc" was 30 cm in diameter and the distance between the microphones was 16.5 cm, which comprises the AB microphone base. The microphones should have ear distance and must be pressure transducers. This system was effectively dubbed the "OSS" technique or, "Optimum Stereo Signal" by its inventor and has found a small community of users who would not wish to compromise this human-based stereo recording technique. The acoustic separation of the baffle disk results in level, time, and frequency response that are called spectral differences. In ear microphones Finally, a pair of specially-designed microphones can be worn by a volunteer, fitted inside their ears, to make use of actual intra-cranial acoustics. Examples Virtual Barbershop